[Qgis-user] Cutting Virtual raster into tiles

[Richard McDonnell]

Hi Bernie,
GdalWarp, or any other Raster manipulation, will indeed will indeed be resampling pixels, when you clip the raster it then tries to fit, align your pixels into your clipped extents.
Could I suggest that you first generate a GeoTiff from your VRT. You can again use GDAL Warp, but the important thing would be to:


1.       Set Georeferenced Extents, using the Calculate from Layer option and set it to your 10km grid File.

2.       Additional command-line… just type –tap this will align all pixels in the output to the crs/grid.

What the above should do is output a TIFF aligned both in extents and pixels to that of your output grid.
I hope that helps, or at least gives you some idea on how you might proceed.

Kind Regards,

Richard






——
Richard McDonnell MSc GIS, FME Certified Professional
Flood Risk Management - Data Management

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Office of Public Works

Sráid Jonathan Swift, Baile Átha Troim, Co na Mí, C15 NX36
Jonathan Swift Street, Trim, Co Meath, C15 NX36
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From: QGIS-User <qgis-user-bounces at lists.osgeo.org> On Behalf Of Bernie Connors via QGIS-User
Sent: 01 November 2024 22:39
To: qgis-user at lists.osgeo.org
Subject: [Qgis-user] Cutting Virtual raster into tiles

Hello,

        I have created a virtual raster of DEM data covering approximately 72,000 km^2.  The DEM has a 1 metre pixel.  The source for the virtual raster is about 95 geoTIFF files with irregular shapes.  I have a SHP file with a 10 km grid of tiles, with a total of 853 polygons (see image below).  I am trying to cut the virtual raster into 853 tiles based on the SHP file.  Each tile will be 10 km x 10 km.  The tiles in the SHP file are exactly 10,000 metres x 10,000 metres.  So I expect the DEM tiles to be exactly 10,000 pixels x 10,000 pixels.  However, when I run the QGIS Easy Raster Splitter plugin the output geoTIFF DEM tiles are 9,999 pixels x 10,000 pixels.  I have checked all 853 tiles and they are all 9,999 x 10,000 pixels.  Here are the first few lines reported by gdalinfo.

Driver: GTiff/GeoTIFF
Files: nb_2015_16_18_dem_2460000_7480000.tif
Size is 9999, 10000
Coordinate System is:

        Here is an image of the 1 metre gap between the 10 km tiles.  I set the scale in QGIS to 1:1,000 for this image.

[cid:image001.png at 01DB2EAB.C6BA1210]

        These 1 pixel (1 metre) gaps appear as vertical gaps with a 10 km spacing when several tiles are displayed together.

        Both the SHP file and the source DEM GeoTIFFs all use the same projected coordinate system - EPSG 2953.  The  Easy Raster Splitter plugin uses gdalwarp in the background.  I suspect gdalwarp is doing some kind of reprojection when it calculates the intersection between the individual tiles in the SHP and the source VRT.  But I am unsure.  Does anybody have a suggestion?
[cid:image003.jpg at 01DB2EAB.C6BA1210]
Thanks,
Bernie.
--
Bernie Connors, P.Eng
B.Sc Geodesy & Geomatics Engineering
Fredericton, NB
bernie.connors at unb.ca<mailto:bernie.connors at unb.ca>
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